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Silicon Cycle in the Tropical South Pacific: Evidence for an Active Pico-Sized Siliceous Plankton

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Silicon Cycle in the Tropical South Pacific: Evidence for an Active Pico-Sized Siliceous Plankton Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-149 Manuscript under review for journal Biogeosciences Discussion started: 15 May 2018 c Author(s) 2018. CC BY 4.0 License. 1 Silicon cycle in the Tropical South Pacific: evidence for an active 2 pico-sized siliceous plankton 3 Karine Leblanc 1, Véronique Cornet 1, Peggy Rimmelin-Maury 2, Olivier Grosso 1, Sandra Hélias- 4 Nunige 1, Camille Brunet 1, Hervé Claustre 3, Joséphine Ras 3, Nathalie Leblond 3, Bernard 5 Quéguiner 1 6 1Aix-Marseille Univ., Université de Toulon, CNRS, IRD, MIO, UM110, Marseille, F-13288, 7 France 8 2UMR 6539 LEMAR and UMS OSU IUEM - UBO, Université Européenne de Bretagne, Brest, 9 France 10 3UPMC Univ Paris 06, UMR 7093, LOV, 06230 Villefranche-sur-mer, France 11 12 Correspondence to : Karine Leblanc ([email protected]) 13 1 Abstract 14 This article presents data regarding the Si biogeochemical cycle during two oceanographic cruises 15 conducted in the Southern Tropical Pacific (BIOSOPE and OUTPACE cruises) in 2005 and 2015. 16 It involves the first Si stock measurements in this understudied region, encompassing various 17 oceanic systems from New Caledonia to the Chilean upwelling between 8 and 34° S. Some of the 18 lowest levels of biogenic silica standing stocks ever measured were found in this area, notably in 19 the Southern Pacific Gyre, where Chlorophyll a concentrations are most depleted worldwide. 20 Integrated biogenic silica stocks are as low as 1.08 ± 0.95 mmol m -2, and are the lowest stocks 21 measured in the Southern Pacific. Size-fractionated biogenic silica concentrations revealed a non- 22 negligible contribution of the pico-sized fraction (<2-3 µm) to biogenic silica standing stocks, 23 representing 26 ± 12 % of total biogenic silica during the OUTPACE cruise and 11 ± 9 % during 24 the BIOSOPE cruise. These results indicate significant accumulation in this size-class, which was 25 undocumented for in 2005, but has since then been related to Si uptake by Synechococcus cells. 26 Our Si kinetic uptake experiments carried out during BIOSOPE confirmed biological Si uptake by 27 this size-fraction. We further present diatoms community structure associated with the stock 28 measurements for a global overview of the Si cycle in the Southern Tropical Pacific. 1 Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-149 Manuscript under review for journal Biogeosciences Discussion started: 15 May 2018 c Author(s) 2018. CC BY 4.0 License. 29 2 Introduction 30 Siliceous phytoplankton, especially diatoms, are often associated with nutrient-rich eutrophic 31 ecosystems. However, the global budget of biogenic silica production by Nelson et al. (1995) 32 already pointed out the importance of these organisms in oligotrophic areas where, despite their 33 low concentration and due to the geographical extension of these systems, their silica production 34 would be comparable to the total for all areas of diatomaceous sediment accumulation combined. 35 However, studies that have documented the Si cycle in the Pacific Ocean, the largest oligotrophic 36 area of the World Ocean, mainly focused on the Equatorial region, and the northern Subtropical 37 gyre. This article presents the first set of field results from the Southern Pacific Ocean between 8 38 and 34° S spanning from New Caledonia over to the Chilean upwelling, and notably, from the most 39 Chl a-depleted region at a worldwide scale (Ras et al., 2008): the South Pacific Gyre (SPG). 40 Diatoms are known to contribute more importantly to primary production in meso- to eutrophic 41 systems, yet several studies have emphasized that even if they are not dominant in oligotrophic 42 regions, they may still contribute up to 10-20 % of C primary production in the Equatorial Pacific 43 (Blain et al., 1997). In the oligotrophic Sargasso Sea, their contribution may be as high as 26-48 % 44 of new annual primary production (Brzezinski and Nelson, 1995) and they may represent up to 30 45 % of Particulate Organic Carbon (POC) export (Nelson and Brzezinski, 1997). In the Eastern 46 Equatorial Pacific (EEP), it has been shown that diatoms experience chronic Si-limitation along 47 the Eastern Equatorial divergence in the so-called High Nutrient Low Silicate Low Chlorophyll 48 (HNLSiLC) system (Dugdale and Wilkerson, 1998) as well as Si-Fe co-limitation (Blain et al., 49 1997; Leynaert et al., 2001). Furthermore, oligotrophic regions are known to experience 50 considerable variability in nutrient injections leading to episodical blooms depending on the 51 occurrence of internal waves (Wilson, 2011), meso-scale eddies (Krause et al., 2010) storms 52 (Krause et al., 2009), or dust deposition events (Wilson, 2003). In nitrogen (N) depleted areas, 53 punctual diatom blooms in the form of Diatom Diazotroph Associations (DDAs) are also known 54 to occur and to contribute both to new primary production (Dore et al., 2008; Brzezinski et al., 55 2011) but also to benefit to non-diazotrophic diatoms through secondary N-release (Bonnet et al., 56 2016; Leblanc et al., 2016). 57 While biogenic silica was classically associated to the largest size fractions, especially 58 microplankton, a series of recent studies have furthermore evidenced a role for picophytoplankton 59 such as Synechococcus in the Si cycle, showing that this ubiquitous lineage is able to take up and 2 Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-149 Manuscript under review for journal Biogeosciences Discussion started: 15 May 2018 c Author(s) 2018. CC BY 4.0 License. 60 accumulate Si (Baines et al., 2012; Ohnemus et al., 2016; Krause et al., 2017; Brzezinski et al., 61 2017). This was evidenced in the field in the Equatorial Pacific, the Sargasso Sea, as well as in 62 culture work, suggesting a widespread diffuse role for this organism, which could be more 63 prominent in oligotrophic environments where diatoms are in low abundance. In the EEP, and 64 despite very variable cellular Si content, Synechococcus represented for instance 40 % of water 65 column biogenic silica (BSi) inventory compared to diatoms in 2004, and twice that of diatoms the 66 following year (Baines et al., 2012). The role of small nano-sized diatoms has also probably been 67 overlooked and we recently pointed out their general occurrence at the worldwide scale and their 68 occasional regional importance in diatom blooms (Leblanc et al. , 2018). 69 Here we present the first set of field results from the Southern Pacific Ocean between 8 and 34° S 70 spanning from New Caledonia over to the Chilean upwelling, and notably, from the most depleted 71 Chl a region worldwide (Ras et al. , 2008), the South Pacific Gyre (SPG). Results were obtained 72 from two cruises carried out a decade apart following longitudinal sections first in the South Eastern 73 Pacific (SEP) between the Marquesas Islands and the Chilean upwelling, crossing the South Pacific 74 Gyre (BIOSOPE cruise, Oct-Dec 2004) and next in the Southern Western Pacific (SWP) between 75 New Caledonia and Tahiti (OUTPACE cruise, Feb-Apr. 2015). Very similar sampling strategies 76 and homogeneous analyses were conducted regarding the Si cycle and provide new data in this 77 under sampled region. We detail size-fractionated BSi inventories in the water column, Si export 78 fluxes, associated diatom community structure composition as well Si uptake and kinetic rates in 79 the Southern Pacific. Our key results show some of the lowest BSi stocks ever measured, which 80 may warrant for a revision of the contribution of oligotrophic areas to the global Si cycle, and 81 confirm recent findings of an active biological uptake of Si in the pico-sized fraction. 82 3 Material and methods 83 3.1 Sampling strategy 84 Results presented here encompass data from two French oceanographic cruises located in the 85 Southern Pacific Ocean (from 10 to 30° S), covering two transects with similar sampling strategies 86 of short and long duration stations. The BIOSOPE (BIogeochemistry and Optics SOuth Pacific 87 Experiment) cruise was undertaken in 2004, while the OUTPACE cruise took place in 2015, both 88 aboard the R/V L’Atalante . The BIOSOPE transect was sampled between the Marquesas Islands 89 (141° W, 8° S) and Concepción (Chile) (72° W, 35° S), between October 24 th and November 12 th 3 Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-149 Manuscript under review for journal Biogeosciences Discussion started: 15 May 2018 c Author(s) 2018. CC BY 4.0 License. 90 2004. The OUTPACE transect was sampled between New Caledonia (159° W, 22° S) and Tahiti 91 (160° W, 20° S) between February 18 th and April 3 rd 2015 (Fig. 1). 92 3.2 Hydrology 93 Water sampling and measurements of temperature and salinity were performed using a SeaBird 94 SBE 911plus CTD/Carousel system fitted with an in situ fluorometer and 24 Niskin bottles. More 95 details about the BIOSOPE cruise strategy are given in the Biogeoscience special issue 96 introductory article by Claustre et al., (2008) while the OUTPACE cruise strategy is detailed in 97 Moutin et al. (2017). Euphotic layer depths (Ze) were calculated as described in Raimbault et al. 98 (2008) and Moutin et al. (2018). 99 3.3 Inorganic nutrients 100 Nutrients were collected in 20 mL PE vials and analyzed directly on a SEAL Analytical auto- 101 analyzer following Aminot and Kérouel (2007) on board during BIOSOPE and at the laboratory 102 during OUTPACE from frozen (-20°C) samples. 103 3.4 Particulate Organic Carbon (POC) 104 Seawater samples (~2 L) were filtered through pre-combusted 25 mm GF/F filters, dried at 60 °C 105 and stored in 1.5 mL eppendorfs PE tubes. Particulate Organic Carbon (POC) was analyzed on a 106 CHN elemental analyzer (Perkin Elmer, 2400 series).
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